Our present invention relates to a switching network of the concentrator-deconcentrator type by which it is possible to connect to an automatic telephone exchange a number of subscribers larger than the total number of signal paths available between the network and the automatic switching equipment of the exchange. To this end, we make use of the well-known fact that not all subscribers use their lines at the same time, so that it is possible to provide only some of them with simultaneous access to the switching network and hence to reduce the size of this network, albeit at the expense of a certain risk of blocking.
The invention also relates to satellites which are independent concentrator-deconcentrator networks disposed at a distance from the exchange for the benefit of a group of isolated and remote subscribers. These satellites optionally provide for local calls to be made between the subscribers of this group without passing through the exchange.
In conventional telephone exchanges, it is known to use for this purpose, for example, rotary selectors whose inputs are connected to the subscriber lines and multiplied between them and whose wipers are connected to the switching network. The more recent versions of exchanges use cross-bar selectors, although the principle of operation is essentially the same.
In electronic time-division telephone switching systems operating with pulse-code modulation (PCM), a concentration system such as this may of course be used, but the arrangement is far from being optimal in terms of dimensions and cost.
In our above-identified application and patent we have disclosed and claimed a new electronic circuit component called a symmetrical time-division matrix (STM). In one particular embodiment, this circuit component enables each of the time slots (TS) of eight incoming PCM junctions of 32 time slots each to be switched to one of the time slots of eight outgoing PCM junctions also comprising 32 time slots each. Under the control of an external signal, internal addressing circuits enable the distribution of the 256 possible space-time routes to be modified. Such matrices can be used for forming the entire switching network of an automatic electronic exchange, irrespective of the size of the network.